Researchers from ICFO use quantum physics to observe delicate systems

A paper published in Nature Photonics introduces a new way to observe very delicate bodies based on quantum physics. Researchers from the Institute of Photonic Sciences (ICFO) have shown that groups of photons organized in certain quantum states can gently explore the properties of objects in a non-invasive way. The results overcome for the first time a limit imposed by quantum mechanics, and may permit the observation of unknown properties of ultra-sensitive objects such as individual atoms or living cells.

Light is used in many fields of science to obtain precise information without damaging the subject matter. In biology, for instance, living cells can be visualized using optical microscopes. The optical microscope uses a photon beam to penetrate the cell and give a picture. Due to the high transparency of the cell, most of the photons pass through without leaving a trace. A small fraction, however, is absorbed and causes cell damage, in much the same way that the sun causes sunburn. Certain cells are more delicate, and the damage caused by photons can be mortal. Until now in these circumstances, scientists have limited the amount of light used in order to avoid damaging the cell, but the cost was a lower quality image.

Quantum physics could dramatically change this scenario. The research group led by ICREA Professor at ICFO Morgan Mitchell has shown that photons prepared in certain quantum states can provide more information, giving a clearer picture while causing less damage. These states are difficult to imagine: they require that all photons be polarized in one direction and also in the opposite direction, being in two different states at once. This is similar to a thought experiment described by Erwin Schrödinger in 1935. He imagined a hypothetical cat in a "superposition of states", being simultaneously alive and dead at the same time. Two years ago, researchers in Mitchell’s group proposed a method for producing these exotic states. In this article, they demonstrate the experimental realization of this quantum state and its increased effectiveness for imaging very delicate objects.

The researchers have chosen a cloud of rubidium atoms for their experiment. This cloud of rubidium serves as a model system: "Atoms of rubidium are a good model because on one hand, they share the same characteristics as the cells in relation to the information obtained and damage caused and, on the other hand, we have very precise knowledge of their characteristics” explains Mitchell.

In the experiment, scientist prepared pairs of photons in a “Schrödinger cat” state, and then sent them through a cloud of atoms, measuring their polarization on the way out. In this way, they were able to deduce the number of atoms in the cloud and the magnetic field of their surroundings. At the same time, they were able to assess the damage, i.e. the number of photons absorbed by the cloud. With the Schrödinger cat, the ratio between information obtained and damage caused exceeds the so-called "standard quantum limit", which quantifies the maximum amount of information obtainable with any traditional probing. "Overcoming this limit provides rigorous proof of the effectiveness of quantum physics for measuring delicate objects" concludes Professor Mitchell.